The roentgen (R) is a measure of how much ionization is produced by radiation. By definition, 1 roentgen is equal to 2.58 x 10 coulomb of free charge produced in one kilogram of air. Approximately 1 roentgen per hour is produced at a 1-meter distance from 1 gram of radium.
The roentgen equivalent mammal (rem) is defined as 1 rem = 1 rad x relative biological effectiveness (which measures biological damage to a given tissue by a certain radiation).
